Why do many Christians assume that non-Christians are unhappy?

Because they (poor, poor souls) can ONLY be happy knowing Christ. They really can't fathom that some of us don't have a huge hole inside us that needs to be filled by Jesus, or don't want to be led around by the nose like sheep, and like to be able to ask questions without being told, "Get out of the garden of Eden!" After all, Eve wouldn't have eaten the apple if God had just explained the premise of knowledge to her and then stuck around to answer her question about it, rather than simply saying, "Don't touch the hot stove, my little four-year olds" and leaving the kitchen before the kids could say, "Why?"

)O(

I've seen that around here from some Christians as well.

It's just that they can't conceive of anyone being happy outside their religion so they blame "sin" and that we're being "willful".

It's a case of being unable to think outside the box they're comfortable with. We're either happy in our "sin", being "deceived by Satan", or lying if we say we're happy and comfortable with our non-Christian beliefs (or lack of beliefs).

Not only do non Christians seem more happy but they are less judgmental and far more tolerant and loving to their fellow man.

surprisingly I have been a director of two charities and and worked with a number of other charities around the world before I retired. The vast majority of volunteers were non christian!!!!!

I think the term you're searching for is fulfillment. Christians have found fulfillment in a relationship with Jesus. Happiness is a brief and fleeting emotion, so certainly anyone can be happy at times and unhappy at other times regardless of whether they are Christian or not. A relationship with Jesus Christ answers the deep longing of the human soul for meaning. Theologians have called it a "God-shaped hole" in the hearts of all humanity. Everyone is longing for fulfillment which is why we try to get more stuff or try different religions/philosophies. I have found it true for myself as have millions of others throughout history.

It's about elitism and exclusion. The more they can disparage atheists, they better they look in comparison.

Here are additional lies that are told about atheists:

1) They are depressed because they don't believe in god.

2) They hate the god in which they don't believe.

3) They have been molested as children or traumatized someway, so that something is psychologically wrong with them to not believe in god.

4) They cannot love, since they do not believe in god.

5) They are angry at the god in which they have no belief.

It's a part of their belief system that no non-believer can be truly happy with out Jesus. I'm not sure if this concept actually has Biblical backing or not, but they accept it as truth regardless of the evidence. This is standard behavior, deny the evidence in favor of belief.

Because they've been taught that all people are unhappy without their god, so they're unable to comprehend happiness outside of their own belief system.

Christians are quite unhappy. They don't get to have any fun.

I disagree,many Christians as well are unhappy,where did you get a notion that just because your a Christian your always happy?

I think because they feel that they find comfort in believing in something like god. But I feel more like they need a crutch and feel bad for them. I'm happier knowing that I don't need that in my life.